The present invention relates to the art of sintering metal injection molded preforms or flowbodies, and more particularly to a two-step sintering process and related tools for controlling flowbody deformation which typically occurs during the sintering process.
Metal injection molding (MIM) is a well known technique for the cost effective production of complex multidimensional parts. Typically such parts are of comparatively small size with a weight within a range of about 25 to about 250 grams and are often made in high production volumes. Metal injection molding is most commonly used in the automotive, firearms, and medical industries.
In general, the MIM process involves mixing a powder metal, water and a binder. The binder is typically composed of an organic aqueous based gel. The mixed powder metal and binder composition produces a generally flowable mixture at relatively low temperature and pressure. The proportion of binder to powder metal is typically about 40-60% binder by volume. The goal is to produce a flowable mixture with a viscosity such that the mixture will fill all of the crevices and small dimensional features of a mold. The flowable mixture is typically transferred to the mold, via an injection molding machine.
Injection molding machines are known in the art and are typically capable of applying several hundred tons of pressure to a mold. The mold is typically constructed with internal cooling passages to solidify the flowable material prior to removal. The mold cavity typically is larger than that of the desired finished part to account for the shrinkage that occurs after binder removal. The mold structure may be formed from either a rigid or a flexible material, such as metal, plastic, or rubber. Preferably, the mold is equipped with vents or bleeder lines to allow air to escape from the mold during the molding process. Alternatively, the mold may be equipped with a porous metal or ceramic insert to allow air to escape from the mold. After the mold has been filled with the flowable mixture, pressure is applied to the mold/mixture to form the molded part, otherwise known as the preform. Typical injection mold pressures for a preform are in the range of about 10-12 ksi. The as molded preforms may be referred to as “green” parts. The green preform may be dried by oven heating to a temperature sufficient to vaporize most of the remaining water. Then, the preform is placed in a furnace to vaporize the binder. To achieve a part with high density and thus a sufficient working strength, the preform is subsequently sintered.
Sintering is an elevated temperature process whereby a powder metal preform may be caused to coalesce into an essentially solid form having the same or nearly the same mechanical properties as the material in casted or wrought form. Generally, sintering refers to raising the temperature of the powder metal preform to a temperature close to, but not exceeding, the melting point of the material, and holding it there for a defined period of time. Under these conditions, interparticulate melting occurs and the material densifies to become solid.
In general, complete solidification does not occur, but sintered density can approach 99% with some materials. As the densification process occurs, the interstitial voids in the preform shrink in size and decrease in number. As a result, the bulk volume of the sintered preform is considerably less than that of the pre-sintered preform. As the preform shrinks, geometric deformation of the preform may occur. This deformation is relatively minor in small parts and can be easily remedied by secondary machining operations. However, in large parts, those with net weights over 250 grams, undesired deformation is more problematic.
In general, during the period of densification, while the preform is subjected to high temperature, preforms of certain configurations, such as tubular or other shapes, have less strength to resist deforming influences and it is a recognized challenge in sintering such metal parts to achieve final geometries congruent to the preform. See, e.g., U.S. Pat. No. 5,710,969. This problem is particularly apparent when sintering preforms with large cylindrical sections and irregular high mass protrusions. For example, a large cylindrical preform section will deform under the influence of gravity to a densified section in the form of an oval. For this reason, the use of MIM and sintering technology has not expanded to the production of comparatively large parts weighing in excess of about 250 grams, or to parts having cylindrical sections with diameters in excess of about 3.8 cm. What is needed therefore is a sintering method and tools which will allow for comparatively larger parts to be sintered while maintaining the geometric stability of the parts.